Extrusion of foamed polymer over diverging mandrel extension

ABSTRACT

1. A PROCESS FOR FORMING A LOW DENSITY FOAMED EXTRUDATES COMPRISING: ADMIXING AN EXPANDING AGENT WITH A THERMOPLASTIC POLYMER; RENDERING SAID THUS FOAMED ADMIXTURE INTO A MOLTEN CONDITION; AND MAINTAINING SAID POLYMER MIXTURE IN SAID MOLTEN CONDITION UNDER SUFFICIENT PRESSURE SO SUBSTANTIALLY NO EXPANDION TAKES PLACE WITHIN AN EXTRUDER; THEREAFTER PASSING SAID MOLTEN ADMIXTURE THROUGH AN ANNULAR CHANNEL OF A DIE HAVING A GRADUALLY INCREASING CROSS-SECTIONAL AREA SO AS TO ALLOW SAID POLYMER TO GRADUALLY EXPAND AS IT MOVES TOWARD THE DIE END; AND UPON EXITING FROM THE DIE GRADUALLY EXPANDING SAID THUS FORMED AND EXPANDED ANNULAR EXTRUDATE RADIALLY OUTWARDLY BY PASSING THE SAME OVER A MANDREL WHICH HAS STRAIGHT WALLS DIVERGING TOWARD A DOWNSTREAM END THEREOF, SAID MANDREL EXTENDING BEYOND AN END OF SAID ANNULAR CHANNEL A DISTANCE EQUAL TO 1 TO 20 TIMES A DIAMETER OF SAID MANDREL, AN OUTER SURFACE OF SAID EXTRUDATE NOT BEING IN CONTACT WITH ANY FORMING SURFACE DURING THIS STEP.

United States Patent Oflice 3,845,184 Patented Oct. 29, 1974 3,845,184EXTRUSION OF FOAMED POLYMER OVER DIVERGING MANDREL EXTENSION Ludo C.Nackaerts, Louvain, Belgium, assignor to Phillips Petroleum CompanyFiled Oct. 24, 1972, Ser. No. 299,833 Int. Cl. B29d 23/04, 27/00; B29f3/04 US. Cl. 26454 ABSTRACT OF THE DISCLOSURE Cellular polymerextrudates are formed by passing molten polymer containing an expandingagent through a die which allows gradual expansion thereof just beforeit leaves the die. The extrudate is then drawn over a graduallydiverging mandrel extension. The process is particularly applicable tothe forming of rigid PVC pipe.

BACKGROUND OF THE INVENTION This invention relates to the formation ofcellular tubular extrudates.

It has long been known to form cellular extrudates. However, thesuccessful commercial production of cellular tubular extrudates,particularly self-supporting extrudates such as pipe, presents manyproblems. With PVC, for instance, temperatures of greater than 190 C.are frequently required which present the dual problem of thermolinstability and low viscosity with its resultant tendency to causebubble collapse. Even processes which seem acceptable on a laboratoryscale may be unacceptable for commercial use because of buildup ofadditives on the extrusion die. Finally, it is difiicult to obtain auniform cellular structure at the lower density levels. With rigid PVC,for instance, densities of less than 0.80 are diflicult to obtain evenwhen using elaborate and expensive modifications to conventionalextrusion equipment.

BRIEF SUMMARY OF THE INVENTION It is an object of this invention toprovide low density cellular tubular extrudates;

It is a further object of this invention to allow fabrication of highquality cellular tubular extrudates with a minimum of modification ofconventional extrusion equipment;

It is yet a further object of this invention to provide formulationswhich allow continuous extrusion of tubular cellular extrudates withoutbuildup of deposits on the die; and

It is still yet a further object of this invention to utilize apparatusfor continuously forming high quality, low density cellular tubularextrudates which can be adapted from conventional extrusion apparatuswith a minimum of modification.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, forming a parthereof, wherein like reference characters denote like parts in thevarious views, FIG. 1 is a cross sectional view of a die head inaccordance with the invention; FIG. 2 is an enlarged view showing thepolymer expanding within the die; and FIG. 3 is a schematic view of anentire apparatus in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is broadlyapplicable to the production of cellular extrudates of any thermoplasticbut is of primary utility in the formation of articles from rigidpolyvinyl chloride. In the most preferred embodiments the rigid PVCformulation can contain small amounts of a conventional plasticizer suchas dioctylphthalate. By small Claims amounts it is meant amounts wellbelow the amount required to make a plasticized formulation. Suitableamounts would be less than 10 percent, preferably 0.25 to 3, morepreferably 0.5 to 2, weight percent based on the weight of the polymer.Other thermoplastic materials normally formed into cellular structuressuch as olefin polymers, particularly polymers and copolymers ofethylene, propylene, and butene, polystyrene, and the like can also befabricated in accordance with the invention.

While any conventional expanding agent can be utilized in accordancewith the invention it is particularly preferred to utilize a formulationcomprising PVC, a chemical blowing agent such as azobisformamide,N,N-dinitrosopentamethylenetetramine, 4,4 oxibis(benzenesulfonylhydrazide), and the like with a combination of a low temperaturepromoter and a high temperature promoter. Suitable low temperaturepromoters are tribasic lead sulfate, monobasic lead stearate, dibasiclead phosphite, dibasic lead stearate, neutral lead stearate and zincstearate. Suitable high temperature promoters are calcium stearate andtin stabilizers. As noted hereinabove a small amount of plasticizer isdesirable. The formulation can also contain fillers and/or nucleatingagents. Fine grade calcium carbonate coated with stearic acid can serveas a filler and nucleating agent.

While the concept of a high and low temperature promoter is broadlyknown, it has been found that in the frame of reference of the instantinvention and the preferred embodiment utilizing a rigid PVCformulation, a greatly superior result is obtained utilizing aparticular ratio of the various components, to wit: chemical expandingagent 0.1 to 0.5, preferably 0.2 to 0.3 weight percent; high temperaturepromoter 0.5 to 1.5, preferably 1.0 to 1.5 weight percent; and lowtemperature promoter 0.05 to 5.0, preferably 1.0 to 2.0 Weight percentbased on the weight of the polymer. Such a formulation having about 2weight percent plasticizer enables the foam to be extruded at a melttemperature of about to C. The incorporation of a material such ascalcium carbonate coated with stearic acid enables continuous operationover long periods of time without any plate-out or uncontrolledexpansion. This material appears to act as a nucleating agent whichgives rise to regular expansion.

An essential feature of the invention is the provision for achieving theexpansion within the die head by providing a zone at the downstream endof the die head which gradually increases in cross-sectional area sothat the expansion can take place within the die. The mandrel extendsbeyond the die face a distance generally within the range of l-20,preferably 2-7 times its diameter, which extension has smooth, straightwalls diverging in the direction of extrusion an amount within the rangeof 0.1 to 5 preferably 0.25 to 1, more preferably about 05, so as toform a smooth surface on the interior of the extrudate and to expand theannular extrudate radially outward.

It has been found that it is possible to produce foamed rigid PVCextrudates of excellent quality having a final density as low as 0.65 orlower, generally within the range of 0.65 to 0.75 g./cc. Extrudateshaving density up to 1.4 g./cc. can also be produced. Since theexpansion essentially all takes place within the die, it can be seenthat the ratio of the annular cross-sectional area at the die face tothe area at the point where the increased cross-sectional area beginsshould be within the range of 1.1:1 to 3:1, preferably about 2:1,although this can be varied dependingon the exact density of thestarting material and the final product density desired.

By tubular extrudate is meant pipe or any other extrudates having acentral bore whether round, square or of irregular shape. Preferablysuch articles have a wall thickness of at least 1 mm. and areself-supporting.

Referring now to the drawings, particularly FIG. 1 there is shown a diehead 12 having a passage way 14 for conveying molten plastic undersufiicient pressure to prevent expansion of a foaming agent. Centrallydisposed within said die head is mandrel 16. The terminal portion ofmandrel 16 as depicted by reference character A, in combination with thesurrounding bushing 18 forms a zone having gradually increasing annularcross section so as to allow said polymer to gradually expand. Theportion of mandrel 16 within zone A may taper inward as shown in FIGS. 1and 2 or may taper outwardly slightly in the direction of flow with thebushing also tapering outward so long as the cross-sectional area ofannulus 20 gradually increases in the direction of extrusion. It isessential to allow gradual expansion of the product within the die asopposed to allowing it to expand suddenly on exiting from the die.Mandrel 16 extends past the die face 22 as depicted by referencecharacter 16a. The mandrel in this zone diverges outwardly a smallamount. The extrudate then passes into sizing sleeve 24 in vacuum watertank 42 where the outside diameter is calibrated.

FIG. 2 shows the thermoplastic polymer-expanding agent mixture 26 undersuflicient pressure that substantially no expansion is taking place withthe expansion occurring in zone A to form the final expanded product 28.Here mandrel 16 is shown having an inwardly tapering configuration 30 inzone A with bushing 18 having surfaces 32 which are cylindrical.

FIG. 3 simply shows an extruder 34 having hopper 36 into which plasticpellets or powder are deposited along with a chemical blowing agentwhich blowing agent is mixed with the polymer as same is rendered into amolten condition in an initial zone 38 of said extruder prior toconveying same through zone 40 where said polymer is heated to atemperature at which the blowing agent is decomposed but is maintainedunder sufficient pressure that substantially no expansion takes place,whereupon the resulting mixture is conveyed to head 12 as shown in theprevious drawings.

Many conventional parts have been omitted from the drawings for the sakeof simplicity but their inclusion is understood by those skilled in theart and is within the scope of the invention.

EXAMPLE The following formulations were prepared, the

amounts being given in parts by weight:

PVC (having a K value of 6070) 100. 100. 0 Tribasic lead sulphate 1. 1.5 Calcium stearate 1. 0 1. 0 Monobasie lead stearate (42% P100)... 0. 50. 5 Stearic avid 0. 1 0. 1 Di-octyl-phthalate (D OP) 0-5 0-5Azodicarbonamide type:

Genitron AC/Z, AC/4 (Fisons) 0. 250.35

EPA (Fisons) 0. 2-0. 3 Coated calcium carbonate 1. 0

These formulations were passed through an extruder of the type shown inthe figures. The mandrel extended beyond the die face approximately 250mm. and had a constant diverging taper of 0.5 degrees. The sectioncorresponding to zone A wherein the expansion takes place had a lengthof 12.5 mm. with the mandrel having a diameter at the initial portion ofthis zone of 52.5 mm. and a diameter at the die face of 51 mm., thebushing have a diameter face of 55 mm. The resulting extrudate passed onover a mandrel extension and into a vacuum sizing tank having a metalsleeve about 12 cm. long to calibrate same and thence into water in saidtank. Final product density was 0.65 g./cc. The product had a final wallthickness of 2 mm. Other runs were conducted giving products having adensity of 0.65 to 0.75 g./cc. The product had a smooth surface on theinside and out and was produced for an extended period of time at highrates of extrusion. The extrusion temperature was varied from 170 to 190C. with good results being obtained at all temperatures.

While this invention has been described in detail for the purpose ofillustration it is not to be construed as limited thereby but isintended to cover all changes and modifications within the spirit andscope thereof.

I claim:

1. A process for forming low density foamed extrudates comprising:

admixing an expanding agent with a thermoplastic polymer;

rendering said thus formed admixture into a molten condition;

and maintaining said polymer mixture in said molten condition undersuflicient pressure so substantially no expansion takes place Within anextruder; thereafter passing said molten admixture through an annularchannel of a die having a gradually increasing cross-sectional area soas to allow said polymer to gradually expand as it moves toward the dieend; and

upon exiting from the die gradually expanding said thus formed andexpanded annular extrudate radially outwardly by passing the same over amandrel which has straight walls diverging toward a downstream endthereof, said mandrel extending beyond an end of said annular channel adistance equal to 1 to 20 times a diameter of said mandrel, an outersurface of said extrudate not being in contact with any forming surfaceduring this step.

2. A method according to claim 1 wherein said polymer is rigid PVC.

3. A method according to claim 1 wherein the ratio of cross-sectionalarea of said molten admixture at the end of said annular channel to thecross-sectional area at the beginning of said annular channel is withinthe range of 1.1:1 to 3:1 and wherein said walls of said mandrel divergean amount within the range of 0.l to 5.

4. A method according to claim 1 wherein said thus radially expandedannular extrudate is passed into contact with a sizing sleeve tocalibrate the outside diameter thereof.

5. A method according to claim 1 wherein said polymer is rigid PVC andsaid expanding agent comprises a chemical blowing agent, a lowtemperature promoter, and a high temperature promoter.

6. A method according to claim 5 wherein said polymer mixture contains0.5 to 2 weight percent of a plasticizer based on the weight of saidpolymer.

7. A method according to claim 5 wherein said polymer mixture contains0.05 to 5 weight percent of said low temperature promoter and 0.5 to 1.5weight percent of said high temperature promoter, said percentages basedon the weight of said polymer.

8. A method according to claim 5 wherein said extrudate is a pipe.

9. A method according to claim 8 wherein said mixture comprises, basedon parts by weight: PVC tribasic lead sulfate 1.5; calcium stearate 1.0;monobasic lead stearate 0.5; stearic acid 0.1; dioctylphthalate 0.5;azodicarbamide 0.25-.35; and stearic acid coated calcium carbonate 1.0.

10. A method according to claim 8 wherein the extrusion temperature isto C. and said pipe has a final density of 0.65 to 0 .75 g./cc.

(References on following page) References Cited UNITED STATES PATENTSBurggren et a1. 425325 XR Lux et a1 26446 Gilbert 26448 XR Winstead264-48 UX Richie 26453 XR Gilbert 26453 Takahashi et a1. 264Dig. 5

6 FOREIGN PATENTS 1,963,811 6/1971 Germany 26448 PHILIP E. ANDERSON,Primary Examiner U.S. Cl. X.R.

161-161; 2602.5 E; 264209, Dig. 5, Dig. 13; 425 325, 467

1. A PROCESS FOR FORMING A LOW DENSITY FOAMED EXTRUDATES COMPRISING: ADMIXING AN EXPANDING AGENT WITH A THERMOPLASTIC POLYMER; RENDERING SAID THUS FOAMED ADMIXTURE INTO A MOLTEN CONDITION; AND MAINTAINING SAID POLYMER MIXTURE IN SAID MOLTEN CONDITION UNDER SUFFICIENT PRESSURE SO SUBSTANTIALLY NO EXPANDION TAKES PLACE WITHIN AN EXTRUDER; THEREAFTER PASSING SAID MOLTEN ADMIXTURE THROUGH AN ANNULAR CHANNEL OF A DIE HAVING A GRADUALLY INCREASING CROSS-SECTIONAL AREA SO AS TO ALLOW SAID POLYMER TO GRADUALLY EXPAND AS IT MOVES TOWARD THE DIE END; AND UPON EXITING FROM THE DIE GRADUALLY EXPANDING SAID THUS FORMED AND EXPANDED ANNULAR EXTRUDATE RADIALLY OUTWARDLY BY PASSING THE SAME OVER A MANDREL WHICH HAS STRAIGHT WALLS DIVERGING TOWARD A DOWNSTREAM END THEREOF, SAID MANDREL EXTENDING BEYOND AN END OF SAID ANNULAR CHANNEL A DISTANCE EQUAL TO 1 TO 20 TIMES A DIAMETER OF SAID MANDREL, AN OUTER SURFACE OF SAID EXTRUDATE NOT BEING IN CONTACT WITH ANY FORMING SURFACE DURING THIS STEP. 